On 02/13/2018 12:15 PM, Jason Merrill wrote:
On Tue, Feb 13, 2018 at 1:31 PM, Martin Sebor <mse...@gmail.com> wrote:
On 02/13/2018 09:24 AM, Martin Sebor wrote:

On 02/13/2018 08:35 AM, Martin Sebor wrote:

On 02/13/2018 07:40 AM, Jason Merrill wrote:

On Mon, Feb 12, 2018 at 6:32 PM, Martin Sebor <mse...@gmail.com> wrote:

While testing my fix for 83871 (handling attributes on explicit
specializations) I noticed another old regression: while GCC 4.4
would diagnose declarations of explicit specializations of all
primary templates declared deprecated, GCC 4.5 and later only
diagnose declarations of explicit specializations of class
templates but not those of function or variable templates.

Hmm, the discussion on the core reflector seemed to be agreeing that
we want to be able to define non-deprecated specializations of a
deprecated primary template.

Yes, that's what Richard wanted to do.  The only way to do it
within the existing constraints(*) is to define a non-deprecated
primary, and a deprecated partial specialization.  This is in line
with that approach and supported by Clang and all other compilers
I tested (including Clang).

To clarify, this approach works for class templates (e.g., like
std::numeric_limits that was mentioned in the core discussion)
and for variable templates.  Functions have no partial
specilizations so they have to be overloaded to achieve the same

Implementations don't treat the deprecated attribute on partial
specializations consistently.

EDG accepts and honors it on class template partial specializations
but rejects it with an error on those of variables.

Clang accepts but silently ignores it on class template partial
specializations and rejects with an error it on variables.

MSVC accepts and honors it on variables but silently ignores it
on class template partial specializations.

GCC ignores it silently on class partial specializations and
with a warning on variables (I opened bug 84347 to track this
and to have GCC honor is everywhere).

This is clearly a mess, which isn't surprising given how poorly
specified this is in the standard.  But from the test cases and
from the core discussion it seems clear that deprecating
a template, including its partial specializations (as opposed
to just a single explicit specialization) is desirable and
already supported, and that the wording in the standard just
needs to be adjusted to reflect that.


[*] Except (as Richard noted) that the standard doesn't seem to
allow a template to be deprecated.  I think that's a bug in the
spec because all implementations allow it to some degree.

One other note.  While thinking about this problem during
the core discussion, another approach to deprecating a primary
template without also deprecating all of its specializations
occurred to me.

1) First declare the primary template without [[deprecated]].
2) Next declare its non-deprecated specializations (partial
   or explicit).
3) Finally declare the primary again, this time [[deprecated]].

Like this:

  template <class T> struct                S;
  template <class T> struct                S<const T> { };
  template <class T> struct [[deprecated]] S<volatile T> { };
  template <class T> struct [[deprecated]] S { };

  S<int> si;             // warning
  S<const int> sci;      // no warning
  S<volatile int> svi;   // warning

This works as expected with Intel ICC.  All other compilers
diagnose all three variables.  I'd say for [[deprecated]] it
should work the way ICC does.  (For [[noreturn]] the first
declaration must be [[noreturn]], so there this solution
wouldn't work also because of that, in addition to function
templates not being partially-specializable.)

My understanding of the reflector discussion, and Richard's comment in
particular, was that [[deprecated]] should apply to the instances, not
the template itself, so that declaring the primary template
[[deprecated]] doesn't affect explicit specializations.  Your last
example should work as you expect in this model, but you can also
write the simpler

template <class T> struct [[deprecated]] S { };
template <class T> struct S<const T> { }; // no warning

With this approach there would be no way to deprecate all of
a template's specializations) because it would always be
possible for a user to get around deprecation by defining
their own specialization, partial or explicit.

I think we need to give users the choice of being able to do
one without the other (in addition to both).  I.e., either of

1) Deprecate a primary and all its uses (including partial and
   explicit specializations).

2) Deprecate just a subset of specializations of a template
   without also deprecating the rest.

An example of (1) is std::auto_ptr or the std::is_literal_type
type trait.  The intent is to remove them from namespace std
someday and providing any specializations for them will then
become an error.

An example of (2) is the std::numeric_limits primary template
that Richard brought up.  That was my understanding of what
he wanted to do but even if that's not what he meant it's
a reasonable use case as well.


Reply via email to